bombd- The R-44 doesn't feature optical SPDIF. Perhaps you meant it's coaxial electrical SPDIF input (via RCA connector)? Or perhaps you were using an optical to electrical SPDIF converter in front of the R-44?
Rerun- the suggestion is to transfer directly from the DAT player to a digital recorder which has a digital input, and then transfer the resulting digital files from the recorder to the computer, instead of recording the digital output stream from the UA-1EX using the computer. Even if both transfers are ultimately made via USB to the computer, the prior is far more robust as it is simply transferring data files. The second is recording a real-time data stream to the and is susceptible to timing and buffer errors on the computer USB bus. Recorders are less likely to have problems with that as they are specifically designed for that purpose, rather than being general purpose machines asked to do this specific task.
To do that, you should be able to use any recorder which features a digital input which is compatible with the DAT player's digital output- so you could use a recorder with an optical input, or if the DAT player you are using also has an electrical coaxial digital out, you can use a recorder which features a coaxial digital input.